1. Field of the Invention
The present invention relates to an extreme ultraviolet light source apparatus outputting an extreme ultraviolet light emitted from plasma generated by irradiating a target with a laser light.
2. Description of the Related Art
In recent years, along with a progress in miniaturization of semiconductor device, miniaturization of transcription pattern used in photolithography in a semiconductor process has developed rapidly. In the next generation, microfabrication to the extent of 70 nm to 45 nm, or even to the extent of 32 nm and beyond will be required. Therefore, in order to comply with the demand of microfabrication to the extent of 32 nm and beyond, development of such on exposure apparatus combining an extreme ultraviolet (EUV) light source for a wavelength of about 13 nm and a reflection-type reduction projection optical system is expected.
As the EUV light source, there are three possible types, which are a laser produced plasma (LPP) light source using plasma generated by irradiating a target with a laser beam, a discharge produced plasma (DPP) light source using plasma generated by electrical discharge, and a synchrotron radiation (SR) light source using orbital radiant light. Among these light sources, the LPP light source has the advantage of obtaining extremely high optical intensity close to the black-body radiation because plasma density can be made higher than the DPP light source and the SR light source. Moreover, the LPP light source has the advantage of obtaining a strong light with a desired wavelength band by selecting a target material. Furthermore, the LPP light source is a point light source which has no electrode located around a luminous point and has a nearly isotropic angular distributions. Therefore, extremely wide collecting solid angle can be acquired. The LPP light source with the above-mentioned advantages has attracted attention as a light source for EUV lithography which requires more than several dozen to several hundred watt power.
In the EUV light source apparatus with the LPP system, firstly, a target material supplied inside a vacuum chamber is irradiated with a laser light to be ionized and thus generate plasma. Then, a cocktail light with various wavelength components including an EUV light is emitted from the generated plasma. The EUV light source apparatus collects the EUV light by reflecting the EUV light using an EUV collector mirror which selectively reflects the EUV light with a desired wavelength component, such as a 13.5 nm wavelength component, for instance. The collected EUV light enters an exposure apparatus. On a reflective surface of the EUV collector mirror, a multilayer coating, with a structure in that thin coatings of molybdenum (Mo) and thin coatings of silicon (Si) are alternately stacked, for instance, is formed. The multilayer coating has a high reflectance ratio (of about 60% to 70%) for the EUV light with a 13.5 nm wavelength.
Here, as mentioned above, plasma is generated by irradiating a target with a laser light, and at the same time, particles (debris) such as gaseous ion particles and neutral particles, and tiny particles (metal cluster) which have not been able to become plasma fly out around thereof from a plasma luminescence point. The debris fly toward surfaces of various optical elements such as an EUV collector mirror located in the vacuum chamber, focusing mirrors for focusing a laser light on a target, and other optical system for measuring an EUV light intensity, and so forth. Therefore, fast ion debris with comparatively high energy erode surfaces of optical elements and damage reflective coating of the surfaces. As a result, the surfaces of the optical elements will become a metal component, which is a target material. On the other hand, slow ion debris with comparatively low energy and neutral particle debris will deposit on surfaces of optical elements. As a result, a layer of a compound of metal, which is a target material, is formed on the surfaces of the optical elements. As a result of the debris entering as mentioned above, the reflective coating of each optical element is damaged or a compound layer is formed on the surfaces of the optical elements, whereby reflectance or transmittance of the optical elements decrease and the optical elements become unusable.
In this respect, Japanese patent application Laid-Open No. 2005-197456 discloses a technique such that debris flying from plasma are trapped by a magnetic field generated inside an optical collecting system by a magnetic field generator when current is supplied to the magnetic field generator. According to this technique, by locating a luminescence point of an EUV light within the magnetic field, ion debris flying from the plasma generated around the luminescence point converge in a direction of the magnetic field by Lorentz force by the magnetic field. As a result, contamination of neighboring optical elements with debris and damages of the optical elements can be reduced.
However, in the above-mentioned Japanese Patent Application Laid-Open No. 2005-197456, because a target nozzle is located on the same axis with a magnetic field direction, fast ion debris moving along the magnetic field collide with the target nozzle. As a result, the target nozzle head will be sputtered by ion collision, whereby a shape of the nozzle head will change. Change of the shape of the nozzle head degrades a position stability of a droplet in a case, for instance, where the target is supplied to the plasma luminescence point as the droplet. Furthermore, the nozzle being sputtered induces another factor of contamination of optical elements such as materials of the nozzle released by the sputtering adhering to the optical elements.
As a technique to solve the above-mentioned problems, for example, Japanese Patent Application Laid-Open No. 2007-207574 discloses a structure with which collision of debris against a nozzle and optical elements located in a direction for supplying droplet is reduced by arranging the nozzle in a direction perpendicular to a magnetic field direction.